Nitrogen oxides (NOx) are typical air pollutants which cause smog, acid rain, photochemical smog by atmospheric photochemical reactions and various secondary pollutants (O2, PAN, etc.) harmful to the human body and contribute to the eutrophication of aquatic ecosystems.
Also, nitrogen oxides are currently regulated emissions produced unavoidably from the combustion of most fossil fuels. As methods for decomposing and removing nitrogen oxides, selective catalytic reduction (SCR) technology is used, which utilizes three-way catalysts for gasoline engines, and NH3 and hydrocarbons, etc., as reducing agents.
In such methods, the efficiency of decomposition of nitrogen oxides decreases rapidly when the nitrogen oxides coexist with other components such as oxygen. In addition, the selective catalytic reduction method that has been put into practical use has problems in that the range of reaction temperature is narrow, the performance of the catalyst is deteriorated due to aging, and set-up and operating costs are high. Thus, research and development have been continuously conducted in order to solve these problems.
Oxygen (O2) is present in exhaust gases in case that a sufficient amount of oxygen is supplied for complete combustion of fossil fuels. In this case, the remaining oxygen greatly increases the consumption of NOx reducing agents or greatly reduces the reduction ability thereof. Thus, there is required a method that adsorbs NOx and then desorbs NOx, thereby preventing the NOx reduction ability of the reducing agent from being deteriorated due to the presence of oxygen and effectively achieving the decomposition of NOx. According to this requirement, the research and development of NOx adsorbing materials have been conducted.
NOx adsorbing materials are based on alkaline earth metal oxides such as BaO. The adsorption capacity of these oxides for NO is significantly lower than that for NO2. For this reason, a catalyst for oxidizing and adsorbing NO needs to be used. As this catalyst, Pt that is an expensive transition metal is mainly used. However, the maximum adsorption capacity of these adsorbing materials is generally only 10˜20 mg-NOx/g-cat at 350° C., and significantly deteriorates as temperature increases or decreases. In addition, because these adsorbing materials are based on NO2-adsorbing components rather than NO-adsorbing components, their ability to decompose NOx in the desorption process is low, so that these adsorbing materials cannot act as effective adsorbing materials.
Patent document 1 discloses the reductive decomposition of nitrogen oxides. However, the patent document relates to the reductive decomposition of nitrogen oxides in case that no oxygen is present in exhaust gases. It is difficult to apply to the case in which oxygen is present.
Patent document 2 discloses a method of decomposing nitrogen oxides using carbon monoxide (CO). However, the patent document has a shortcoming in that, when oxygen is present, CO corresponding to the equivalent of oxygen is additionally required.
In addition, patent document 3 adopts a method of supplying CO resulting from partial oxidation of CH4 and discloses a method of decomposing N2O and NOx using an MMO catalyst. However, in order to decompose lean NOx and N2O, a method different from patent document 3 is required.
If oxygen is present in excess, it can be present in an amount much larger than that of NOx, which is at most several thousand ppm or less, and thus the consumption of CO for removing this oxygen will be very high. In this case, NOx treatment costs will be increased due to the consumption of the reducing agent, it will be difficult to find a suitable method for supplying CO, and the amount of CO2 discharged can also be problematic.
Patent Document 1: Korean Patent Registration No. 563903
Patent Document 2: Korean Patent Registration No. 654885
Patent Document 3: Korean Patent Registration No. 638835